Nova 4 – Process engineering

Urine treatment
and fertilizer production.Urine contains high levels of nutrients – and these should not be
allowed to enter waterbodies. One attractive option, therefore, could be to
recover nutrients for use as fertilizers in agriculture. Novaquatis studied a
broad range of processes – biological, chemical and physical – both for the production
of fertilizers and for the removal of nutrients. It was shown that the various
processes are suitable for different purposes and, in most cases, are not energy-intensive.
This means that treatment can be adapted to meet specific requirements.

Research
background

The bulk
of the nutrients from human metabolism are excreted in urine – in particular, nitrogen
(N), phosphorus (P) and potassium (K). These nutrients are desirable in agriculture,
but not in waterbodies (where only K causes no harm). It may therefore make
sense to separate urine from wastewater and use it for fertilizer production.

Fresh urine
is slightly acidic, with a pH of 6–7. However, the high concentration of biologically
degradable substrate promotes rapid bacterial growth. As a result, the chemical
composition of urine undergoes significant changes during collection and
storage. Since urea is hydrolysed to ammonia and carbon
dioxide, the pH rises sharply – to more than 9 (cf. Nova 2). In
addition, urine contains organic micropollutants, especially pharmaceutical
residues and hormones, which are equally unwelcome in waterbodies and in agriculture
(Nova 5).

The
various treatment processes serve widely differing purposes: urine can be
stabilized and its volume reduced; nitrogen and phosphorus can be recovered or
removed; and bacteria, viruses and micropollutants can be eliminated [1]. However,
it is not possible to achieve all the different objectives using a single
process; a decision is thus required as to what is desirable and what is
necessary.

In
general, urine treatment may involve biological (Nova 4-1), chemical (Nova 4-2,
4-3) or physical (Nova 4-3) processes. The advantages and disadvantages of the various
methods are discussed in detail in [1].

Nova 4-1: Biological processes – stabilization

(Kai
Udert, Tove A. Larsen, Willi Gujer)

Nova 4-1 was
concerned with the development of a biological process for urine stabilization [2].
Bacteria cultured in a reactor not only decompose organic compounds in urine
but also convert a portion of the ammonium to nitrite or nitrate (nitrification).
This leads to the production of acid, which lowers the pH of the urine from
more than 9 to about 6, preventing losses of ammonia. At the same time, the
biological processes eliminate unpleasant odours.

With this
process, a solution of ammonium nitrate
or ammonium nitrite is obtained. The
nitrogen compound ammonium nitrate
is a commercial fertilizer. Ammonium nitrite,
in contrast, is toxic to soil organisms. However, it can readily be converted
to nitrate through chemical oxidation with oxygen at a low pH value [3] or,
using another biological process, to a harmless nitrogen gas and water [2].

Biological
reactor for stabilization (Photo Yvonne Lehnhard)

Nova
4-2: Chemical processes – phosphorus precipitation

(Mariska
Ronteltap, Max Maurer, Willi Gujer)

The chemical conditions in stored urine (i.e. high pH values) promote
the precipitation of phosphorus in the form of poorly soluble phosphorus-containing
salts. This leads to encrustation and clogging of pipes (Nova 2). However, the
process can also be used for phosphorus recovery.

With controlled addition of magnesium, phosphorus can be recovered in
the form of struvite
(MgNH4PO4, magnesium ammonium phosphate, MAP). This is attractive,
as two significant wastewater nutrients (P and N) are thus transformed into a
single solid product, which is, moreover, a well-established slow-acting multicomponent
fertilizer.

Nova 4-2
investigated in detail the process whereby struvite is produced from urine [4].
It was shown that the rate of phosphorus elimination depends crucially on the
degree of dilution, but generally reaches 98%. The product obtained is largely
free of pharmaceuticals and hormones, and no heavy metals could be detected [5].

Although
struvite can be used directly as a fertilizer, it is not suitable for further
processing in the phosphorus industry [1]. In a Novaquatis follow-up project, other
precipitation products are being studied that would be suitable for further
processing of this kind. Thus, both options can be kept open.

Nova
4-3: Physical processes – membrane technology

(Wouter Pronk, Markus Boller)

Nova 4-3
considered various urine treatment scenarios, focusing on membrane technologies.
The aims of these methods are threefold: (1) to separate organic micropollutants
from nutrients, (2) to concentrate the nutrient solution (volume reduction) and
(3) to remove or destroy bacteria and viruses. In addition, micropollutants can
also be eliminated via the chemical process of ozonation.

The membrane
technology of nanofiltration was tested in the laboratory. The process is only
effective if urea in fresh urine is not hydrolysed. If this can also be
successfully prevented in practice – e.g. through acidification – nanofiltration
can be used to produce a urea solution (without phosphorus). This solution is largely
unproblematic: a large proportion of the organic micropollutants can be
separated from the nutrients, and bacteria and viruses are eliminated [6]. In
the nanofiltration process, the nutrients are not concentrated – in a further project,
vacuum evaporation was employed for this purpose. With this process, the volume
of a urea solution was reduced by 90% at 78°C [1].

Also
tested in the laboratory were the membrane-based process of electrodialysis and
the chemical process of ozonation. With the aid of electrodialysis, micropollutants
can be largely separated from ammonium, phosphorus and potassium, as can microorganisms
such as bacteria. At the same time, the nutrient solution is concentrated
roughly fourfold [7]. If ozonation is additionally performed, the fertilizer
produced is highly likely to be acceptable as regards both hygiene and contamination
with pharmaceuticals and hormones.

In a
follow-up project, electrodialysis and ozonation are being tested on a pilot
scale for the treatment of urine collected at the Basel-Landschaft cantonal library in Liestal [8] (cf. Nova PP). The nutrient solution produced here contains
12 g N, 0.65 g P and 5.7 g K per litre.

Conclusions

The wide
variety of urine treatment processes available offers substantial flexibility. For
example, if a rural setting calls only for stabilization, to prevent the release
of ammonia when fertilizer is applied, a one-step biological treatment should
be sufficient. But if nutrients are to be recycled in a metropolis – as would
be advisable in areas with a general lack of fertilizers – the demands are higher,
and various processes will need to be combined. Nutrients can, however, also be
eliminated – e.g. to protect sensitive receiving waters from excessive nutrient
loads.

All the
processes will require further development before they can be implemented in
practice. But thanks to the Nova 4 research, we now know precisely what
processes are currently available, for what purposes they are suitable, and in
what respects they need to be optimized.

In many
cases, separate removal or recycling of nutrients is preferable to the
existing practice. This also applies to the energy requirements associated with
these processes [1, 9].